In U.S. Pat. Nos. 3,260,656 and 4,076,596 is presented information concerning principles of operation of prior art sensors, detectors, methods, etc. for detecting the concentration in fluids of an electrochemically active species. The entire disclosures of such patents hereby are incorporated by reference. As used in such patents and herein "fluids" includes gases, liquids, vapors, mixtures thereof, and virtually any other material in which an electrochemically active species may occur and/or be detected.
For example, in the past, detection of the concentration of oxygen as an electrochemically active species in a material was performed by measuring current flow developed by oxidation or reduction reactions. The present invention contemplates such oxygen concentration detection and also detection of other electrochemically active materials by oxidation of reduction current generated technique and/or other reactions that generate an electrical parameter that can be measured, such as current or voltage. Primarily, though, electrochemically active species ordinarily means any group of identical chemical entities, such as ions, molecules, atoms, etc., which are capable of being separated by electrochemical type reaction, such as oxidation or reduction, or other reaction, to yield an electrical parameter, such as current or voltage, that may be detected as a representation of the concentration, for example, of such species.
A conventional approach to measuring oxygen concentration has been to place two electrodes 1, 2 (as seen in FIG. 1), one a working electrode (the cathode, for example of gold, platinum or other noble metal or carbon) and the other a reference electrode (the anode, for example of silver, silver-silver chloride or other material), in an electrolyte 3 having the species, the concentration of which is to be measured. A voltage from source 4 is applied across the electrodes causing a reduction reaction at the cathode, an oxidation reaction at the anode, while maintaining a charge balance in the electrolyte. Exemplary equations for the reduction and oxidation reactions are shown, respectively in equations 1 and 2 below. ##STR1## The current flowing between the electrodes 1, 2 would result from the reactions occurring in the electrolyte; and such current would have a proportional relationship to the concentration of the electrochemically active species and could be measured by a meter 5. Such method is disclosed, for example, in the above mentioned patents.
More specifically, in the '656 patent the cathode and anode, for example of noble metal, are placed in an electrolyte of potasium hydroxide, the combination being within a selectively permeable barrier or membrane that permits passage only of the electrochemically active species therethrough. In such patent the cathode is solid material and the anode is mesh material; and those types of electrodes may be used also in the present invention. Use of gold, platinum, silver, silver oxide, silver-silver chloride electrode materials are mentioned. The patentee also describes the possible problem of pH changes due to consumption of H.sup.+ and suggests adding a buffer for prolonged operation. The '656 patent further shows a three electrode package, including a so-called consuming electrode cathode, an anode, and a reference electrode, and an energization circuit using an amplifier to apply a voltage between the cathode and reference electrodes while the potential of the reference electrode is fixed at a virtual ground.
In the '596 patent there is disclosed a technique for placing an anode and cathode of an oxygen sensor on an electrically non-conductive, inert, for example, plastic, material substrate using a thick film technique and/or a thin film technique. Sputtering and/or evaporation is used to deposit a thin film of metal on the substrate to form the electrode surfaces. Definition of the surface areas is achieved by photo-etching. Use of a further separate reference electrode like the third electrode shown in the '656 patent, also is mentioned. U.S. Pat. No. 4,062,750 is another example of thin film technique used to form an electrochemical electrode and cell.
A number of problems have been encountered in the aforementioned types of electrochemically active species sensors. These include inaccuracies caused by measuring the current flow resulting from the reactions occurring at both the working and reference electrodes and caused by changes encountered as the reference electrode material itself is consumed as well as inaccuracies due to the effect of IR voltage drop in the electrolyte between the two electrodes. For example, the current flow between the two electrodes when a voltage is applied thereacross would be a function of the reduction and oxidation reactions occurring, respectively, simultaneously at the electrodes; whereas only one of those reactions directly proportionally represents species concentration. Therefore, in the past the current density at the reference electrode has been minimized while the relative current density at the working electrode has been increased by relying on a convention to size the electrodes such that the reference electrode has about fifty times the surface area of the working electrode. This size relationship has been beneficial in reducing, although not eliminating, the relative affect on the measured current of current due to the forced reaction occurring at the reference electrode and in intensifying the affect on or proportion of the measured current of current due to the reduction reaction, for example, occurring at the working electrode. Such size relationship also results in the reference electrode itself being so large that small consumption thereof would not detrimentally affect operation of the sensor; therefore, sensor longevity and accuracy during life will be improved over a sensor with about equal size electrodes.
Unfortunately, the just mentioned fifty to one size convention creates a restriction on the miniaturization possibility for the sensor. Yet, it is desirable to provide such sensors of a variety of sizes, from rather large to measure electrochemically active species, for example in commercial or industrial processes, to rather small sensors, for example for biomedical purposes.